1. Field of the Invention
The present invention relates to an echo canceler, more particularly to an echo canceler that detects echo path changes.
2. Description of the Related Art
Echo cancelers are used in telecommunication systems to remove far-end echo from the signal transmitted from the near end to the far end. One type of echo canceler includes an adaptive filter, the tap coefficients of which are updated by a mathematical algorithm to adapt to changes in the echo path. Known algorithms work well when only the far-end party is talking, but fail badly in the double-talk state when both parties talk at once. Conventional echo cancelers therefore include a double-talk detector, and suspend the adaptation process in the double-talk state.
Unfortunately, double-talk detectors are apt to misinterpret changes in the echo path as double-talk, causing adaptation to be suspended just when it is most needed. This is particularly true when the echo path changes in a way that reverses the polarity of the echo signal. While the suspension lasts, since the adaptive filter cannot adapt to the ongoing changes in the echo path, a persistent echo is heard at the far end.
A double-talk detector that detects changes in the echo path by correlating the echo replica signal produced by the adaptive filter with the near-end input signal and incorporates the correlated result into the double-talk detection process is disclosed by Fujii et al. in ‘Double-Talk Detection Method with Detecting Echo Path Fluctuation’, Trans. of IEICE of Japan, Vol. J78-A No. 3, pp. 314-322 (1999, in Japanese). Under an assumed constant level of non-vocal background noise, this echo canceler can recognize even low levels of double-talk and distinguish them from changes in the echo path. The correlation method employed by this echo canceler is unreliable, however, in the presence of vocal background noise such as office noise.
It would be desirable to have an echo canceler that can accurately detect echo path change and control the updating of adaptive filter tap coefficients accordingly.